Reflective stimulus for computer screen simulation for eye examinations

ABSTRACT

A test card for use in eye examinations emulates the light return profile of a video display screen such as those commonly used in connection with computers. The test card is used for prescribing corrective lenses for a computer user. The test card is formed on an opaque planar medium and has a plurality of characters formed thereon for reading by the patient, each character formed by a plurality of pixel elements. The pixel elements may be formed by designing a pattern of dots on a computer system using graphic software for representing the resulting pattern in a machine-readable graphics file. The graphics file is used to control a compatible imagesetter machine for forming the desired pattern on photographic film. According to an alternative method, a film master is prepared in which each pixel is a solid black dot. The test card is prepared by printing the image onto photographic paper through a diffusion element so as to create an approximately Gaussian light return profile across each pixel.

RELATED APPLICATION DATA

This is a division of U.S. patent application Ser. No. 08/148,693, filedNov. 5, 1993 now U.S. Pat. No. 5,440,360 which is a continuation-in-partof co-pending U.S. patent application Ser. No. 08/000,211 filed Jan. 4,1993, now U.S. Pat. No. 5,325,136 which is a continuation-in-part ofU.S. patent application Ser. No. 07/665,903, filed Mar. 7, 1991, nowU.S. Pat. No. 5,191,367 which is a continuation-in-part of U.S. patentapplication Ser. No. 07/282,596, filed Dec. 12, 1988, now U.S. Pat. No.4,998,820.

FIELD OF THE INVENTION

The present invention relates to vision testing equipment and, morespecifically, to a test card for use in conducting optometricexaminations, and to methods of making such apparatus.

BACKGROUND OF THE INVENTION

As use of the video display terminal ("VDT") has become more widespread,for example in connection with computers, so too have certainophthalmological problems associated with its use become more common. Arecent survey of optometrists, reported in the J. Am. Optore. Assoc.1992 (vol. 63, pp 687-92), shows that more than 14% of optometricpatients present with symptoms primarily associated with use of the VDT,or almost 10 million examinations annually when projected to the U.S.population. Responding optometrists were unable to confidently arrive ata diagnosis and treatment more frequently for VDT patients (20.87%)compared to non-VDT patients (14.05%).

Alphanumeric characters displayed on video display screens are made upof dots or "pixels" which do not have well-defined edges and thereforeare difficult for the eye to focus upon. Further, since video screensare maintained at a constant distance of about 50 cm from the user'seyes, the same eye muscles are in constant use in focusing on thescreens. These factors cause significant amounts of stress and fatigueon the eyes of VDT users which are often aggravated by the fact thatmany such users utilize their computers for extended periods on a dailybasis. The stress associated with video display use frequently resultsin peculiar types of eye problems requiring special correctiveprescriptions in the spectacles selected for the users suffering fromthese problems.

In order to accurately diagnose these problems, appropriate testequipment and test procedures must be provided. In accordance with theprocess currently used by medical practitioners to determine thespectacle requirements of typical patients, an apparatus (phoropter) isplaced in front of the eyes of the patient which enables the doctor torapidly change a wide selection of lenses while the patient views a setof test images through the lens changing apparatus. As the patientfocuses on the test images, the doctor assesses the status of themuscles inside the patient's eyes and judges their degree of relaxationthrough the use of a retinoscope. The doctor determines the combinationof lenses and the prescription best suited to the patient by changingthe lenses until he detects the combination which provides the mostrelaxed state in the eye muscles of the patient.

As may be understood from the above, the fitting of corrective lenses isbasically a trial and error process in which the doctor observes thereaction of the patient's eye muscles to an appropriate test image forvarious combinations of lenses. However, without a test image whichaccurately simulates the conditions under which the patient mayexperience eye problems, a prescription for suitable corrective lensesmay not be reliably determined. When presented with an image which doesnot have sharply defined edges or which is slightly out of focus, theeye will respond by reverting to a level of tonic muscle activity knownas the resting point of accommodation, having a focal length with asharply defined image placed at the same distance from the eye.Accordingly, the prescription required for eyeglasses used with a videodisplay terminal can differ from a prescription for use in viewingprinted material.

A vision tester apparatus is disclosed in commonly-owned U.S. Pat. No.5,191,367. That apparatus, although quite useful for its intendedpurpose--computer screen simulation for optometric examinations--iscostly to manufacture. It also is susceptible to various mechanical orelectrical failures. The need remains, therefore, for providing eye carepractitioners with equipment for VDT-user examinations that isinexpensive, reliable and easy to maintain.

SUMMARY OF THE INVENTION

One aspect of the present invention is an optometric test card forreading by a patient during an eye examination. The test card preferablycomprises an opaque, generally planar medium such as stiff photographicpaper. A plurality of characters such as alphanumeric characters areformed thereon for reading by the patient. Each character is formed by aplurality of pixel elements or "dots" arranged in a predeterminedpattern. Each pixel element exhibits a maximum density adjacent itscenter, and the density gradually decreasing as a function of distancefrom the center so that the edges of the pixel elements are "soft" orblurred. The dots are sized so that individual dots are not discernableby the patient at a viewing distance greater than approximately 15inches. As a result, the characters appear fuzzy to the patient, therebysimulating the visual characteristics of a typical computer displayscreen. The test card may include an aperture extending therethough, theaperture sized and arranged so as to allow an examiner to view thepatient's eyes generally along the patient's line of sight by lookingthrough the test card from a side of the test card opposite the patientwhile the patient reads the characters on the test card.

Another aspect of the invention is a method of making the test carddescribed. The method includes selecting a plurality of symbols orcharacters for viewing by a patient and forming an image of the selectedsymbols on a single substantially opaque substrate for presentation tothe patient. Preferably, the step of forming the image includes reducinga density of each symbol along its peripheral edges relative to amaximum density within the corresponding symbol, so that the edges ofthe symbols will appear fuzzy to the patient, thereby simulating thevisual effect of a computer display screen for the purpose of anoptometric examination.

The effect of reducing the density along the edges of the symbols isachieved by forming each symbol out of a plurality of pixel elements.Each pixel element is formed of a plurality of dots, arranged in apredetermined pattern having a maximum dot density per unit areaadjacent the center of the pattern, the dot density gradually decreasingas a function of distance from the center. The dots are sized so thatindividual dots would not be discernable by a patient at a viewingdistance greater than approximately 15 inches (38 cm.), a typicalviewing distance for a computer screen display. The test card may beeconomically produced in volume using known photographic processes.

The foregoing and other objects, features and advantages of theinvention will become more readily apparent from the following detaileddescription of a preferred embodiment which proceeds with reference tothe drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of a test card according to the presentinvention.

FIG. 2 is an enlarged view of a portion of the test card of FIG. 1.

FIG. 3 is a further enlarged view of an individual pixel element of thetest card of FIG. 1 to illustrate gradations in optical density.

FIG. 4 is graph representing light amplitude level as an approximatelyGaussian function of distance across a diameter of the pixel element ofFIG. 3.

FIG. 5 illustrates an array of black dots arranged to form a pixelelement.

FIG. 6 is graph representing light amplitude level as an approximatelyGaussian function of distance across the pixel element of FIG. 5.

FIG. 7 is a cross-sectional view of an arrangement of materials forforming an image on a photo-sensitive medium according to the presentinvention.

FIG. 8 is graph representing light amplitude level as an approximatelyGaussian function of distance across a pixel element formed on aphotographic paper using one method of the present invention.

FIG. 9 illustrates a portion of an image master showing a characterformed of a plurality of dots prior to diffusion printing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An optometric test card 46 is illustrated in front view in FIG. 1. Thetest card image is formed from an opaque, generally planar medium suchas stiff photographic paper. An aperture 52 extending through the testcard may be provided. The aperture is sized and arranged so as to allowan Examiner to view the patient's eyes generally along the patient'sline of sight by looking through the test card from a side of the testcard opposite the patient (the back side) while the patient reads thecharacters formed on the test card. FIG. 2 is an enlarged view of one ofthe characters ("A") formed on the test card of FIG. 1. This shows howeach character 10 is formed of a plurality of pixel elements 12 arrangedin a predetermined pattern so as to form the desired character. Eachpixel element has a maximum density adjacent its center, and the densitygradually decreases as a function of distance from the center, so thatthe edges of the characters appear fuzzy to the patient. This effectsimulates the output of a typical computer display screen.

In particular, the density of each pixel element 12 is an approximatelyGaussian function of distance from the center of the pixel element. FIG.3 is a further enlarged view of an individual pixel element 12 of thetest card of FIG. 1 to more clearly illustrate gradations in opticaldensity. In FIG. 4, a plot 20 shows light amplitude level versushorizontal distance across the pixel of FIG. 3. It may be observed thatthe light amplitude level gradually decreases from an initial or ambientlevel indicated by dashed line 24, along an inverted bell curve to acusp or minimum 28 corresponding to the center of pixel 12 where thedensity is at a maximum (black). Continuing horizontally across thepixel, the light amplitude level then gradually increases along curve 30till it reaches the maximum level 24. The pixel element 12 and the lightamplitude plot 20 preferably are symmetric. The light amplitude curve 20is somewhat idealized or smoothed in that the pixel element actuallycomprises a series of discrete concentric bans, each having a differentdensity. Accordingly, a more exact plot of light amplitude level wouldexhibit a staircase effect showing a step change in light amplitudelevel at the transition between each of the concentric bans, althoughthe overall inverted bell shape of the curve is accurate. This lightamplitude characteristic is selected to most closely emulate the pixelsof a computer display screen in which light is transmitted from a pointsource.

A practical method of making a test card of the type described calls forusing a plurality of discrete dots clustered together so as to form eachpixel element. Thus, each pixel element comprises an array of dotshaving a center, in which the density of discrete dots is maximizedadjacent the center of the array and the dot density gradually decreasesas a function of distance from the center of the array, as furtherexplained below. The dots are sized so that individual dots are notdiscernible by a patient at a viewing distance greater than anapproximately 15 inches (38 cm), a typical computer display screenviewing distance. Since the individual dots are not discernible, thearray or cluster simulates an individual pixel of a computer displayscreen.

Referring now to FIG. 5, a preferred array of dots 32 is illustrated.Each individual dot (for example dot 38) is disposed on one of a seriesof concentric circular rings (for example rings 34, 36). Thisarrangement is convenient for creating a cluster of dots having thedesired density distribution. Each individual dot is substantiallycircular and has a diameter on the order of 0.001 inch (0.025 mm). Thesedimensions are far too small to carry out the method manually. Twomethods of creating the test cards are disclosed herein, the imagesettermethod and the diffusion method. The imagesetter method involves the useof a computer and suitable software for generating film work masters ona machine that converts electronic layout files to photographic film.The electronic file that provides an original image for this method maybe created, for example, in a software program called "AdobeIllustrator", made by Adobe Systems, Inc. Release version 3.2.3 was usedin the examples discussed herein.

Using the computer, each pixel element is formed of a cluster or groupof black circular dots, on the order of 0.001 inch in diameter. The dotsare arranged in concentric circles, with the spacing of dots increasingas a function of distance from the center. In one example of a preferredembodiment, the center of the pixel is coincident with the center pointof the first dot. Next, six 0.001 inch dots are positioned with theircenters spaced apart at 60° angles on a circle of 0.0019 diameter havingits center at the center of the pixel. Continuing outward, twelve dotsare placed with their centers spaced 30° on a circle of 0.0038 inchdiameter. The placements of dots continues as shown in the followingtable.

    ______________________________________    Centerline Circle Dia.                    Number of Dots on Circle    ______________________________________    .0057"          18    .0076"          19    .0095"          20    .0113"          19    .0133"          15    .0151"          12    .0171"          11    .0180"          11    .0183"          11    .0210"          11    ______________________________________

This arrangement is illustrated in FIG. 5. A cluster of dots formedaccording to this arrangement exhibits a light amplitude level as afunction of horizontal distance as indicated by the curve 42 in FIG. 6.The electronic file thus created is converted to photographic film by animagesetter machine using, for example PostScript® page descriptionlanguage. In the operative example, the imagesetter used was aColorSetter®, manufactured by Optronics Corporation. The output film is0.007" Kodak negative photographic film, with the emulsion layerright-reading on the rear surface. This provides a master film image.Production duplicates of the master are created by standard photographiccontact duplication, using Kodak QCP resin-coated negative photographicpaper or a similar product. This results in production images havingblack characters on a white background. The same characters can becreated using Kodak QCP resin-coated positive photographic paper, toresult in production images with white characters on a black background.The black characters on the white background are shown as panel 48 inFIG. 1.

The concentric circles, for example circles 34, 36, merely illustratethe placement of the individual dots. The circles themselves are notpart of the image. The arrangement of the dots described exhibits thedesired Gaussian light amplitude levels, and is reasonably uniform atthe viewing distances of interest.

The production images produced on photographic paper may be mounted to asubstrate of stiffer material. The test card, thus reinforced, may bemounted to a bracket and suspended from a typical ophthalmic chairreading rod.

The overall effect of the preceding arrangement of dots into pixels isseen when viewed from a distance on the order of 20". The viewer's eyecannot resolve the individual dots, and the pixels are perceived ashaving poorly-defined or fuzzy edges. This acts to drive the viewer'sfocus to their resting point of accommodation, which can be measured bythe eye care practitioner. The test card apparatus and method of theinvention thus provide a simple and inexpensive means for simulatingcomputer display screens for ophthalmic examination.

Diffusion Method

An alternative method of making a test card that involves the use of thesame computer hardware and software described above, and the step ofgenerating a film work master on an imagesetter. However, according tothe alternative method, the film work master consists of a square-waveimage, with the Gaussian characteristics imparted during the process ofduplication onto photographic paper.

In one example of an operative embodiment, to generate the diffusionmaster, the Adobe Illustrator® computer program is used to locate 0.003"diameter dots arranged in predetermined patterns in a 7×9 matrix so asto create desired alphanumeric characters. These dots are spacedapproximately 0.012" to 0.015" on center, similar to the spacingpreferred for the pixel method described above. However, in this casethe dots are solid black, as distinguished from the fuzzy pixel elements12 of FIG. 2. FIG. 9 illustrates the solid dots arranged as described soas to form the letter "A". Such characters may be arranged into wordsand sentences as illustrated in FIG. 1. Arrangement of the charactersinto actual readable text is preferred so that the patient actuallyreads the test card, just as the patient in practice actually reads thetext on a computer display screen.

Next, the electronic file provided by the Adobe Illustrator® is convenedinto photographic film by an Optronics imagesetter using PostScript®page description language, as noted above. The output film again may be0.007" Kodak negative photographic film, with the emulsion layerfight-reading on the rear surface. This provides a master film image.Note, however, that in this case larger (0.003" diameter) dots are usedin lieu of the clusters of smaller dots described above for forming apixel element.

Production duplicates of this master image are created by standardphotographic contact duplication, but with the addition of a spacerlayer introduced between the emulsion layer of the film master and thefront surface of the Kodak QCP resin-coated negative photographic paper.

Referring to FIG. 7, a cross-sectional view is shown of an apparatus forforming an image on a photo-sensitive medium according to this diffusionmethod. In the figure, light energy indicated by dashed arrows 54 isdirected through the photographic negative film master 56. Eachindividual dot on the master results in a 0.003" diameter opening, forexample opening 58 in the emulsion layer of the film master.Accordingly, light energy indicated by dashed arrows 62 is transmittedthrough the opening and dispersed in the diffusion layer 60. Thediffusion layer preferably is formed of a polyester spacer, on the orderof 0.011" thick. A sheet of resin-coated negative photographic paper 64is positioned in parallel contact with a side of the polyester spaceropposite the film master. The spacer disperses the light, as indicatedby dashed arrows 62. As a result, a solid black dot in the master imagethat formed opening 58 in the film creates a corresponding diffusedimage 66 on the surface of the photographic paper 64. Because of thediffusion employed, image 66 will have a maximum density at its center,so that light amplitude level is at a minimum, and the light amplitudewill vary as a function of horizontal distance across the image in asubstantially Gaussian function as illustrated by curve 70 in FIG. 8.The resulting photographic paper may be mounted to a stiffer substrateas described above.

Having illustrated and described the principles of my invention in apreferred embodiment thereof, it should be readily apparent to thoseskilled in the art that the invention can be modified in arrangement anddetail without departing from such principles. We claim allmodifications coming within the spirit and scope of the accompanyingclaims.

What is claimed is:
 1. An optometric test card for reading by a patientduring an eye examination comprising:an opaque, generally planar mediumhaving a plurality of characters formed thereon for reading by thepatient, each character formed by a plurality of pixel elements arrangedin a predetermined pattern; each pixel element having a center, andhaving a maximum optical density adjacent the center, the densitygradually decreasing as a function of distance from the center, so thatthe edges of the characters appear fuzzy to the patient, therebysimulating the visual effect of a computer display screen and whereinthe medium comprises a photosensitive material and the characters areformed thereon by a photographic process.
 2. An optometric test cardaccording to claim 1 wherein the optical density of each pixel elementis an approximately Gaussian function of distance from the center of thepixel element.
 3. A method of making an optometric test cardcomprising:selecting a plurality of symbols for viewing by a patient;and forming an image comprising the selected symbols on a singlesubstantially opaque substrate for presentation to the patient; saidforming step including: providing a photographic material; and formingthe image by selectively exposing the photographic material; whereinsaid forming step includes reducing an optical density of each symbolalong its peripheral edges relative to a maximum density within thecorresponding symbol, wherein said reducing step includes forming eachsymbol of a plurality of pixel elements as provided by a computerdisplay screen, so that the edges of the symbols will appear fuzzy tothe patient, thereby simulating the visual effect of a computer displayscreen in a photographic print for the purpose of an optometricexamination.